The present invention relates to aircraft turbofan engines in which an annular fan casing carries an inner, abradable liner that surrounds the fan rotor. More particularly, the present invention relates to a fan casing liner that is supported by a fan outer casing to allow radial expansion and contraction of the liner independently of radial expansion and contraction of the fan outer casing.
Aircraft gas turbine engines of the turbofan type are characterized by a relatively large diameter axial-flow fan that is positioned ahead of the relatively smaller diameter compressor of a core engine. The fan provides an initial pressure increase to the air that enters the engine air inlet and before the air enters the compressor. Because the fan has a larger diameter than that of the core engine compressor, a part of the air that exits from the fan passes around the outer periphery of the compressor and flows over Fe outer casing of the core engine. Generally, the airflow from the fan that bypasses the core engine is a considerable volume of the inlet air stream, ranging, for example, from about five to about eight times the airflow through the core engine.
Because of the large mass of bypass air that exits from the fan, and because of the higher pressure of the bypass airflow, relative to the inlet airflow, the thrust output available from turbofan engines is increased considerably for a relatively small increase in fuel consumption. Consequently, the fuel usage efficiency of such engines is greater than that of conventional turbojet engines that do not involve such a bypass arrangement. In addition, the propulsive efficiency of turbofan engines is greater than that of turbojet engines because the high mass of bypass stream airflow is discharged at a lower velocity than that of the smaller mass of air and combustion products that are discharged at a considerably higher velocity in a turbojet engine.
For high-bypass-ratio turbofan engines the radially-extending fan rotor blades are considerably longer than conventional axial-flow compressor rotor blades. And to maintain a high operating efficiency of the fan section of the engine, the clearance between the radially outermost ends of the fan blades and the inner surface of the surrounding fan casing must remain small over a wide operating range of the engine to avoid fan blade tip losses. Such losses result when a portion of the air is allowed to flow over the tips of the fan blades, rather than axially, with the result that the pressure ratio between the fan inlet and the fan outlet is diminished, resulting in decreased engine thrust.
Fan blades utilized in high-bypass-ratio turbofan engines are sometimes made from composite materials to reduce fan blade weight while maintaining blade strength and aerodynamic efficiency. Fan casings, on the other hand, are generally made from metals to provide a sufficiently strong containment structure to retain fan blades and blade fragments that might separate from the fan rotor, and to prevent them from being thrown outwardly through the fan outer casing. Composite materials of the type utilized in making fan blades have a relatively low coefficient of thermal expansion, while the metallic fan casing material, usually an aluminum alloy, has a significantly higher coefficient of thermal expansion. Accordingly, because of the differences in the thermal expansion characteristics of the fan blade and of the casing material, it is difficult to maintain the desired small blade tip clearance for highest fan aerodynamic efficiency. In that regard, the variation of blade tip clearance solely from thermal effects over the extremes of an aircraft operating envelope can be as much as 0.140 in.
Additional factors affecting blade tip clearance are the loads imposed on the fan blades during aircraft maneuvers and during times in which rapid output demands are placed on the engine. Thus, during rapid engine acceleration, or during certain aircraft maneuvers, centrifugal forces acting on the blades can cause blade elongation in a radial direction that can result in contact of the blade tips with the inner surface of the casing. Accordingly, fan casings generally include abradable materials on the inner surface of the fan casing to avoid direct contact between the fan blades and the fan casing, and to allow for removal by the fan blade tips of part of the abradable material to accommodate the differences in thermally-caused expansion and loading-caused expansion of the fan blades and of the fan outer casing.
In addition to the effects of thermal changes and aircraft maneuver load changes on fan blade length, the fan casing of a turbofan engine must be capable of withstanding the loss of a fan blade. Such a condition can occur as a result of the impact with a blade of a foreign object, such as a bird or a piece of ice that forms at the engine air inlet and that subsequently becomes dislodged from the inlet and is drawn into the fan by the inlet airflow to strike a blade. When the loss of a fan blade occurs the fan rotor becomes unbalanced, thereby imposing larger radial loads on the fan rotor bearings. Additionally, the tips of the remaining blades describe a larger diameter circle by virtue of the orbital movement of the fan rotor axis as a result of the unbalanced condition of the fan rotor. Consequently, the tips of the remaining blades dig into the abradable material, which is rigidly attached to the inner surface of the fan casing, thereby scraping away a considerable portion of the material and possibly causing blade tip contact with the metallic outer fan casing.
One way in which thermal expansion differences between the fan blade material and the fan casing material can be reduced is by extracting pressurized air from the core engine to cool the fan casing to limit its thermal expansion. But that approach adds weight to the engine, and it also reduces the airflow through the core engine, thereby reducing engine thrust output.
It is therefore desirable to provide a fan casing structure that will maintain efficient fan aerodynamic performance over a wide range of fan component operating temperatures, and that will also allow fan rotor orbiting to occur in the event of a fan rotor unbalance condition caused by the loss of a fan blade or of a portion of a fan blade.
Briefly stated, in accordance with one aspect of the present invention, a casing is provided for surrounding a rotatable member having a plurality of substantially radially extending blades. The casing includes an annular outer shell and an annular inner liner carried within and spaced inwardly of the outer shell and substantially coaxial therewith. A plurality of radially-inwardly-extending support members are carried by the outer shell and extend at least partially into the inner liner to support the inner liner against relative axial and rotational movement, and to allow relative radial movement therebetween.